Melting phase relations in the systems Mg2SiO4–H2O and MgSiO3–H2O and the formation of hydrous melts in the upper mantle

Novella, Davide; Dolejš, David; Myhill, Robert; Pamato, Martha G.; Manthilake, Geeth; Frost, Daniel J.

doi:10.1016/j.gca.2016.12.042

Cited by 13 publications

(13 citation statements)

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“…An inspection of the equilibrated liquid structure and radial distribution functions (RDF) g ( r ) shows a homogeneous phase even for fo27H 2 O. This observation is supported by two experimental inferences: (i) Novella et al (2017) suggested that Mg 2 SiO 4 + 20.4 wt% H 2 O is a single liquid phase at high P and T based on quench textures of their samples; (ii) phase relations of the peridotite‐H 2 O system (22–63 wt% H 2 O) studied by in situ X‐ray radiography (Mibe et al, 2007) indicate that at P > 4 GPa the miscibility gap between silicate melt and fluid is closed; that is, hydrous silicate melt and aqueous fluid in the deep upper mantle become indistinguishable from each other. However, our observation of a homogeneous phase is not sufficient to claim that fo27H 2 O is thermodynamically stable: Phase separation rarely occurs spontaneously in regular DFT‐MD simulations, a problem that partly arises from the fact that an interface between two phases is not stable in small systems (Hong & Van De Walle, 2013).…”

Section: Computational Detailsmentioning

confidence: 66%

“…As water preferentially partitions into melts compared to solid peridotite residues (Aubaud et al, 2004;Novella et al, 2014), partial melts of hydrous material from the MTZ are expected to be water-rich (up to 16.5 wt% H 2 O), supported by observations from melting experiments (Freitas et al, 2017;Litasov & Ohtani, 2002;Zhang et al, 2017) and insight from thermodynamic modeling (Hirschmann et al, 2009;Novella et al, 2017). Whether these water-rich melts stay at depth-and therefore provide a viable explanation for the geophysical observations-is an open question, as they appear to be far too water-rich to be neutrally buoyant.…”

Section: Introductionmentioning

confidence: 87%

“…While DFT-MD simulations on the silica-H 2 O system (Spiekermann et al, 2016) did explore water-rich compositions (23 wt% H 2 O), the vast majority of previous theoretical investigations have focused on probing the behavior of silicate-H 2 O systems containing hydrogen with water contents typically not exceeding 10 wt % (e.g., Bajgain et al, 2015;Karki & Stixrude, 2010;Mookherjee et al, 2008). Here we go beyond this H 2 O content, following the suggestions of high water concentrations in silicate melts formed near the MTZ (Freitas et al, 2017;Hirschmann et al, 2009;Novella et al, 2017), and perform DFT-MD simulations on hydrous forsterite (Mg 2 SiO 4 ) melts with 16.6 wt% and 26.7 wt% H 2 O and compare the results to those of a more typical composition considered in simulations with 5.5 wt% H 2 O. With these simulations, we explore whether H 2 O has a polymerizing effect on depolymerized silicate melts as proposed by previous experiments (Mysen & Cody, 2005;Xue & Kanzaki, 2004;Yamada et al, 2007Yamada et al, , 2011, and we constrain the influence of H 2 O on melt densities-and therefore buoyancy-in the MTZ and its vicinity.…”

Section: Introductionmentioning

confidence: 99%

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Structure and Density of H₂O‐Rich Mg₂SiO₄ Melts at High Pressure From Ab Initio Simulations

2020

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Water has a strong effect on silicate melt properties, yet its dissolution mechanism in depolymerized melts, typical for mantle composition, remains poorly understood. Here we report results of first-principles molecular dynamics simulations for hydrous Mg 2 SiO 4 melts with 6, 16, and 27 wt% H 2 O at pressure and temperature conditions relevant to the upper mantle and mantle transition zone. The results show that hydrogen bonds not only to the network-forming cation Si but also to Mg which-nevertheless-remains the most important network modifier. There is no evidence to support the hypothesis based on experimental data that water may cause an increase in melt polymerization for ultramafic magmas; the ratio of non-bridging oxygen per Si increases with the addition of the oxygen from H 2 O. The partial molar volume of water is independent on concentration in our simulations which allows us to examine the density of hydrous melt systematically. The critical water content-at which melts are neutrally buoyant compared to the surrounding mantle-is~4 wt% H 2 O for a pyrolite composition, much lower than the high water content (>10 wt%) observed in petrological experiments and estimated thermodynamically for low-degree partial melts formed in the vicinity of the mantle transition zone. Plain Language Summary Geophysical observations indicate abrupt changes in the material properties of the Earth's mantle right below and above the transition zone, at a depth between 410 and 660 km. The existence of low-velocity and high conductivity layers near this region is often taken as an indication for melting caused by the presence of water. However, the density of such hydrous melts is not well characterized over the relevant pressure-temperature range. Here we present new simulations on Mg 2 SiO 4 melts with a wide range of water concentrations to address this issue. The water content at which hydrous melts have the same density as the surrounding mantle is about 4 wt%, much lower than the high water content (>10 wt%) estimated for melts occurring in the Earth's mantle, suggesting they cannot experience long-term gravitational stability in the mantle transition zone and its vicinity. We further find that water does not cause a significant change in melt structure as proposed previously.

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Section: Computational Detailsmentioning

confidence: 66%

Section: Introductionmentioning

confidence: 87%

Section: Introductionmentioning

confidence: 99%

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Structure and Density of H₂O‐Rich Mg₂SiO₄ Melts at High Pressure From Ab Initio Simulations

2020

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“…Into each disk were spark-eroded two rows of three holes, each 250 microns in diameter and 700 microns deep. The capsules 90 can be found in Frost et al (2004) and Keppler and Frost (2005 Novella et al, 2016). The experiments were quenched by cutting power to the heater.…”

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confidence: 99%

Hydrous melting and partitioning in and above the mantle transition zone: Insights from water-rich MgO–SiO2–H2O experiments

Myhill

Frost

Novella

2017

Geochimica et Cosmochimica Acta

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General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Abstract Hydrous melting at high pressures affects the physical properties, dynamics and chemical differentiation of the Earth. However, probing the compositions of hydrous melts at the conditions of the mantle transition zone has traditionally been challenging. In this study, we conducted high pressure multianvil experiments at 13 GPa between 1200 and 1900 • C to investigate the liquidus in the system MgO-SiO 2 -H 2 O. Water-rich starting compositions were created using platinic acid (H 2 Pt(OH) 6 ) as a novel water source. As MgO:SiO 2 ratios decrease, the T-X H2O liquidus curve develops an increasingly pronounced concave-up topology. The melting point reduction of enstatite and stishovite at low water contents exceeds that predicted by simple ideal models of hydrogen speciation. We discuss the implications of this work on the behaviour of melts in the deep upper mantle and transition zone, and present new models describing the partitioning of water between the olivine polymorphs and associated hydrous melts.

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“…(a)Hydration of the Earth's upper mantle by diffusion. The estimations are based on the diffusion coefficient of 7.89×10 -9 m 2 /s(Novella et al, 2017a) at 1723K with initial H2O…”

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confidence: 99%

Electrical conductivity of hydrous silicate melts: Implications for the bottom-up hydration of Earth's upper mantle

Freitas

Manthilake

2019

Earth and Planetary Science Letters

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The upwelling of the hydrous mantle transition zone triggers dehydration-induced partial melting atop the 410-km discontinuity. Here we investigate the electrical conductivity of hydrous silicate melts in the 200-400 km depth range and explore whether melting at the 410-km depths is responsible for the hydration of the upper mantle. Our experimental electrical conductivity data demonstrate that the mantle at 180-350 km depths is mostly melt free, confirming the H2O under-saturated conditions. However, the residual mantle from partial melting atop the 410km discontinuity may contain various possible amounts of water according to the initial mantle transition zone and melt concentrations. This residual H2O could contribute to the hydration of the upper mantle either through diffusion or material replacement by upwelling. Our calculations suggest that the diffusion may not be responsible for the hydration of the upper mantle to present H2O concentration of 50-200 ppm wt. Melting of the upwelling mantle transition zone with less than 1500 ppm wt. H2O produces residual peridotites with ~ 200 ppm H2O at the 410-km discontinuity. Continuous upwelling of such hydrous residues would gradually replace the dry upper mantle with depleted residual hydrous peridotites in less than 260 Ma. In this study, we propose a bottom-up hydration mechanism for the Earth's upper mantle driven by dehydration-melting at the 410-km

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Melting phase relations in the systems Mg2SiO4–H2O and MgSiO3–H2O and the formation of hydrous melts in the upper mantle

Cited by 13 publications

References 65 publications

Structure and Density of H₂O‐Rich Mg₂SiO₄ Melts at High Pressure From Ab Initio Simulations

Structure and Density of H₂O‐Rich Mg₂SiO₄ Melts at High Pressure From Ab Initio Simulations

Hydrous melting and partitioning in and above the mantle transition zone: Insights from water-rich MgO–SiO2–H2O experiments

Electrical conductivity of hydrous silicate melts: Implications for the bottom-up hydration of Earth's upper mantle

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Melting phase relations in the systems Mg2SiO4–H2O and MgSiO3–H2O and the formation of hydrous melts in the upper mantle

Cited by 13 publications

References 65 publications

Structure and Density of H2O‐Rich Mg2SiO4 Melts at High Pressure From Ab Initio Simulations

Structure and Density of H2O‐Rich Mg2SiO4 Melts at High Pressure From Ab Initio Simulations

Hydrous melting and partitioning in and above the mantle transition zone: Insights from water-rich MgO–SiO2–H2O experiments

Electrical conductivity of hydrous silicate melts: Implications for the bottom-up hydration of Earth's upper mantle

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Structure and Density of H₂O‐Rich Mg₂SiO₄ Melts at High Pressure From Ab Initio Simulations

Structure and Density of H₂O‐Rich Mg₂SiO₄ Melts at High Pressure From Ab Initio Simulations